The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gases in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process fluid flow transmitter provides an output related to a sensed process fluid flow. The flow transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled.
Measuring the rate of fluid flow in a confined conduit by modifying the internal geometry of the conduit and applying an algorithm to the measured differential pressure in the flowing fluid, is known. The geometry of the conduit is traditionally changed by altering the cross section of the conduit, such as with a venturi meter, or by the insertion into the conduit of a flow altering device such as a orifice plate, or an averaging pitot tube or the like.
An averaging pitot tube generally includes a shaped bluff body that slightly impedes fluid flow within the conduit. One limitation of some averaging pitot tubes is a relatively lower signal to noise ratio in the differential pressure data being sensed. "Noise" in the context of a differential pressure measuring device, such as a flow transmitter, is the instantaneous deviation from an average pressure reading from one data point to another. The noise generated in a pitot tube type of differential pressure sensor originates in the impact pressure sensors on the upstream facing side of the pitot tube and in the non-impact pressure ports generally on the downstream side of the pitot tube.
As differential pressure transmitters and data acquisition systems have become more sophisticated and responsive, they have also become more sensitive to and are increasingly influenced by the noise generated by the pressure sensing unit. Accordingly, the noise characteristics of differential pressure sensing devices, have become a more important factor in their selection and operation. Thus, there is a need to provide an improved differential pressure sensing device having an improved signal to noise ratio.